Too Much Nitrogen

Posted July 25, 2017byMark Macdonald

25 Jul

Nitrogen is one of the three most important so-called “macronutrients” for the healthy growth of plants, along with its equally important cousins phosphorus and potassium. These three elements are the central components of most fertilizers, and they form the N-P-K ratio that is shown on fertilizer packaging. A good, balanced, organic fertilizer for general garden use might have an N-P-K ratio of 4-4-4, as does the outstanding All Purpose Blend from Gaia Green.

One of the most common problems we hear about in the garden, and not just from beginners, is the presence of too much nitrogen. This element is fundamental to the growth of leaves and plant tissue, and an important food for the many soil organisms that convert other nutrients into forms that are available to plants. Nitrogen is also a component of chlorophyll, so it is central to basic photosynthesis, and therefore needed by all plants. A lack of nitrogen might result in plants that were stunted and yellowy, with withered growth and overall poor health.

However, when too much nitrogen is present, what tends to result is an explosion of foliar growth, but at the expense of flower formation, fruit set, and root growth. It’s not uncommon to hear about really vigorous beets or carrot tops, where the vegetables produce lush, unruly, abundant leaves, but no root to speak of. We have heard about pea plants that seem to race skyward, but then produce few flowers that are followed by disappointingly few peas.

How does it happen?

Gardeners and growers like to prepare their soil at the start of the season by applying mulch, manure, cover crops, and/or fertilizer. For a crop of sunflowers (for instance) to grow twelve feet tall, with massive leaves and a giant seed head in just three or four months, the plant needs to draw an amazing amount of nutrients from the soil. Whatever crop is harvested at the end of the season, masses of soil nutrients come with it. So gardeners and growers have a genuine need to cultivate healthy, fertile soil for good crop results.

Some of those soil amendments and fertilizers can be excessively high in nitrogen. One common example is animal manure that has not been fully composted. We have heard of people planting into pure, undiluted Sea Soil. Some organic soil amendments like feather meal (a byproduct of chicken and turkey rendering), has an N-P-K ratio of 15-0-0, so it is pretty much pure nitrogen. Chemical nitrogen is also available in the form of ammonium nitrate — this highly unstable chemical was once the go-to form of fertilizing commercial fields, but that’s a bit off topic… Usually, it is the application of some sort of well-intended soil amendment that produces the problem of excess nitrogen.

How to fix it?

Of all three macronutrients, nitrogen is the quickest to become depleted in soil. Water from winter rain and snow washes a tremendous amount of it away. De-nitrifying bacteria consume available nitrogen and further deplete soil. And heavy agricultural use of soil also sucks out much of the available nitrogen.

Perhaps the best way to take advantage of a soil that is discovered to be nitrogen heavy is to simply plant crops that thrive on nitrogen. Leafy greens of all sorts, including nearly all crops that are not harvested for roots (like carrots), shoots (like broccoli), or fruits (like peppers & beans), need nitrogen more than they need phosphorus and potassium. Kale, pac choi, mustards, lettuce, spinach, and most chicories would be good candidates for nitrogen rich soil.

By contrast, it’s those crops that produce roots, shoots, and fruits that are the most likely to struggle when nitrogen levels are excessive.

The notion of trying to bring the three macronuntrients into balance by bringing up the phosphorus and potassium levels would likely cause more problems than it would solve. A preferable method would simply be to let the soil rest. Apply a mulch of organic material, like leaves, and give the soil some time to come back into a natural balance. Then, when it’s time to plant again, be sure that any fertilizer inputs have that balanced ratio like the 4-4-4.